^Them it would appear that the Mac 30 is not capable of adapting to the variable impedance swings of typical loudspeakers without deviating from linear frequency response?
No, not at all, unless the amp is set up on the wrong tap.
If the speaker is 4 ohms in the bass and 8 or 16 ohms in the highs, you put it on the 4 ohm tap and the bass will be correct as well as the highs. It will be just as flat as a transistor amp on the same speaker.